Did you mean English-English or some other language like English-German (which has a lot less entries than the English-English one).
Also you should have a look at the metadata and source code of your book. If the book language is not set to English, or of the text has some tags in it (ruby tags for japanese, or span tags, shy hyphenation marks, etc.) it can throw the dictionary off.
So how well dictionary lookups work, also depends on the book itself, unfortunately.
|